The chemical nature of the labile selenium moiety in erythrocyte glutathione peroxidase will be investigated. Low molecular weight, selenium derivatives released from the 75Se-labeled enzyme spontaneously or by treatment with inhibitory reagents will be identified using column chromatography, and thin layer electrophoresis and chromatography. Mass spectral studies of suitable derivatives will also be conducted. Ultraviolet spectroscopy will be used to obtain information relating the structure of selenium in various redox states of the enzymes to the relativity of that form with specific inhibitors. Further studies of heavy metal-selenium interactions will be performed to learn more about the mechanism of the protective effects of selenium with special reference to methylmercury. The adequacy of selenium in complete therapeutic diets will also be assessed to see if use of such diets may lead to a suboptimal selenium status in humans. BIBLIOGRAPHIC REFERENCES: Ganther, H.E., Hafeman, D.G., Lawrence, R.A., Serfass, R.E., and Hoekstra, W.G. "Selenium and Glutathione Peroxidase in Health and Disease: A Review" in Chap. 32 of Trace Elements in Human Health and Disease, vol. II, eds. A.S. Prasad and D. Uberleas, Academic Press, N.Y., 1976. Hsieh, H. Steve, and Ganther, H.E. "Effects of Stock or Purified Diet on Rat Liver Enzymes Involved in the Synthesis of Dimethyl Selenide." J. Nutrition (1976), in press.